Fix groupby on float/double NaN values. Previously NaNs were being split into separate groups since the standard >/< comparisons don't behave properly for NaNs. Now for float/double columns we check and handle NaNs directly.

KOLAP-1999

Fix incorrect template parameter

KOLAP-1999

Fix nan handling in groupby. Cuda 8 evidently doesn't like custom comparison functors in thrust::stable_sort_by_key. Instead of using a custom comparator now we 'lexicographically' sort by first sorting by the vector itself (but setting nan values to 0), then sorting by a vector of is_nan values

KOLAP-1999

Fix sorting to be nan-aware (the nan groupby test was failing on certain GPUs and/or cuda versions). NaNs will sort higher than non-NaNs